The exhaust gas of vehicles using gasoline as fuel contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and the like. Each of the harmful components is desired to be purified by a catalyst such that the hydrocarbons (HC) are converted into water and carbon dioxide by oxidation, the carbon monoxide (CO) is converted into carbon dioxide by oxidation, and the nitrogen oxides (NOx) are converted into nitrogen by reduction.
Three way catalysts (TWC) which can oxidize and reduce CO, HC and NOx are used as such a catalyst to treat the exhaust gas (hereinafter, referred to as the “exhaust gas purifying catalyst”).
As such three way catalysts, those obtained by supporting a refractory oxide porous material having a great specific surface area, for example, an alumina porous material having a great specific surface area with a precious metal such as platinum (Pt), palladium (Pd) and rhodium (Rh) and then supporting a substrate, for example, a monolithic substrate composed of a refractory ceramic or a metal honeycomb structure with this or supporting refractory particles with this are known.
In the three way catalysts, the precious metal has a function to convert the hydrocarbons in the exhaust gas into carbon dioxide and water by oxidization and to convert the carbon monoxide into carbon dioxide by oxidization but to reduce the nitrogen oxides to nitrogen. It is preferable to preserve a ratio of air to fuel (air fuel ratio) at a constant value (at the theoretical air fuel ratio) in order to effectively cause the catalytic action for both of these reactions at the same time.
The air fuel ratio of the internal combustion engine of vehicles and the like greatly changes depending on the driving conditions such as acceleration, deceleration, low speed driving, and high speed driving, and thus the air fuel ratio (A/F) which fluctuates depending on the operating conditions of the engine is constantly controlled by using the oxygen sensor. However, it is impossible for the catalyst to sufficiently exert a performance as a purifying catalyst only by controlling the air fuel ratio (A/F) as described above, and thus the catalyst itself is also required to have an action of controlling the air fuel ratio (A/F). Hence, a catalyst obtained by adding a promoter to a precious metal which is a catalytically active component is used for a purpose to prevent a decrease in the purification performance of the catalyst caused by the change in the air fuel ratio by the chemical action of the catalyst itself.
As such a promoter, a promoter (OSC material) is known which exhibits the oxygen storage capacity (OSC) to release oxygen in a reducing atmosphere and to absorb oxygen in an oxidizing atmosphere. For example, ceria (cerium oxide, CeO2) and a ceria-zirconia composite oxide are known as the OSC material exhibiting the oxygen storage capacity.
Ceria exerts the oxygen releasing and absorbing property in association with a change in valence (trivalent tetravalent), and a change in valence of ceria is further promoted by using zirconia in the form of a solid solution, and thus the technology using a ceria-zirconia composite oxide is adopted as the mainstream technology of the recent exhaust gas purifying catalysts.
With regard to the ceria-zirconia composite oxide, for example, a catalyst is disclosed in Patent Document 1 (JP 2005-296735 A) which is obtained by supporting iron oxide on a carrier containing a ceria-zirconia composite oxide.
In addition, a catalyst is disclosed in Patent Document 2 (JP 2004-160433 A) which is composed of a composite oxide of at least one kind of metal selected from the group consisting of ceria, zirconia, aluminum, titanium, and manganese, and iron.
A catalyst is disclosed in Patent Document 3 (JP 2008-18322 A) which has a configuration in which iron oxide is dispersed in a ceria-zirconia composite oxide so as to at least partially form a solid solution.
A ceria-zirconia composite oxide is disclosed in Patent Document 4 (JP 2009-84061 A) in which the regularly arranged phase of the pyrochlore phase type is still present at 50% or more after heating for 5 hours at a temperature condition of 1000° C. in the air.
Moreover, an OSC material is disclosed in Patent Document 5 (JP 2011-219329 A) which is a ceria-zirconia-based composite oxide containing a composite oxide of ceria and zirconia in which the content ratio of cerium to zirconium in the composite oxide is in the range of 43:57 to 48:52 by molar ratio ([cerium]:[Zirconium]) and the phase separation of ceria is suppressed, that is, which contains a fluorite type ceria-zirconia composite oxide and a pyrochlore type ceria-zirconia composite oxide.